1. Field of the Invention
The present invention relates to a method of manufacturing a laminated ring for use as a belt in transmitting power in a continuously variable transmission, and a molten salt composition for use in such a method.
2. Description of the Related Art
Continuously variable transmissions have a power transmitting belt trained around a pair of pulleys. The power transmitting belt comprises a laminated ring composed of a plurality of rings and mounted on and held by an element of predetermined shape.
The laminated ring is straight in shape when traveling between the pulleys, and curved in shape when running along the pulleys. The laminated ring undergoes severe bending deformations due to repetitive cycles of the straight and curved states. Therefore, the laminated ring is required to have a mechanical strength large enough to withstand the severe bending deformations. Heretofore, it has been customary to produce a laminated ring by welding the opposite ends of a sheet of maraging steel to form a cylindrical drum, severing the cylindrical drum into rings, and rolling the rings to a predetermined length.
The maraging steel is a low-carbon steel containing 17 to 19% of Ni, and Co, Mo, Ti, etc. When the maraging steel is heated to a suitable temperature after being subjected to a solution treatment, it causes age hardening in a martensitic state, resulting in an ultra-high strength steel that is highly strong and highly tough. The maraging steel is highly suitable for use as the material of the laminated ring used under severe conditions because of the high strength and the high tenacity.
In as much as the maraging steel should desirably have wear resistance and fatigue-resistant strength for use as the material of the laminated ring, it is the customary practice to perform case-hardening on the maraging steel.
For example, the maraging steel is case-hardened by effecting a gas nitriding or gas soft-nitriding process on the rolled rings. However, since the gas nitriding or gas soft-nitriding process alone is not effective enough to activate the surface of the steel, the process needs to be preceded by a solution treatment which heats the rings to a temperature range from the recrystallization temperature of the maraging steel to 830xc2x0 C. When the rings are heated to the above temperature range, circumferential lengths of the rings are required to be corrected to eliminate heat-treatment strains caused by the heating and an aging process is needed to stabilize a temperature distribution in a subsequent nitriding process.
Therefore, the gas nitriding or gas soft-nitriding process is disadvantageous in that it requires a complex process after the rolling process, and also that gas nitriding or gas soft-nitriding process itself is time-consuming.
One solution is to replace the gas nitriding or gas soft-nitriding process with a salt-bath nitriding process. In the salt-bath nitriding process, the rings are dipped in a molten salt containing potassium cyanate (KCNO) or sodium cyanate (NaCNO) that is produced when a major component of potassium cyanide (KCN) or sodium cyanide (NaCN) reacts with air. The salt-bath nitriding process is referred to as tufftriding process named after the tradename name of the molten salt.
The molten salt contains, in addition to KCN or NaCN and KCNO or NaCNO, potassium carbonate (K2CO3) or sodium carbonate (Na2CO3), with CNO in the range from 31 to 35% and CN in the range from 1 to 2%. The molten salt is used while being heated to a temperature in the range from 570 to 580xc2x0 C. The salt-bath nitriding process using the molten salt activates the surface of the ring due to decomposition of KCNO or NaCNO and nitrides of the surface of the ring with nitrogen produced by the decomposition of KCNO or NaCNO. The salt-bath nitriding process can be finished in a period of time much shorter than the gas nitriding or gas soft-nitriding process. The salt-bath nitriding process can be performed on the rings immediately after the rings are rolled, without the need for the solution treatment, the circumferential length correction, and the aging process which are required in the case of the gas nitriding or gas soft-nitriding process.
One problem with the salt-bath nitriding process is that because the nitriding process progresses in a short period of time, the thickness of nitride layers formed in the surfaces of the rings changes greatly per unit time, making it difficult to obtain nitride layers of appropriate thickness. If the thickness of the nitride layers is too small, then a desired level of wear resistance and fatigue-resistant strength cannot be achieved. If the thickness of the nitride layers is too large, then an age-hardened layer inwardly of the nitride layer is reduced, failing to obtain a desired level of strength.
The rings exhibit such a tendency that their volume tend to increase in their surfaces because of the formation of the nitride layers, and tend to decrease inwardly wardly of the surfaces because of the formation of the age-hardened layer. As a result, the rings suffer dimensional changes such as circumferential length changes in order to keep the structure stable. According to the salt-bath nitriding process, the circumferential length of the rings tend to increase as a whole because the expansion of the surface layers due to the formation of the nitride layers is greater than the shrinkage that occurs inwardly of the surface layers.
It is desirable that the thickness of nitride layers formed by the salt-bath nitriding process be controlled appropriately.
To solve the above problems, the present applicant has proposed a process for dipping rings in a salt bath in which a molten salt containing 31-35% of CNOxe2x88x92 and 1-2% of CNxe2x88x92 is heated to 520-530xc2x0 C., for a period of time ranging from 10 to 25 minutes (see Japanese laid-open patent publication No. 10-121130). According to the proposed process, the temperature of the salt bath is lower than the conventional temperature ranging from 570 to 580xc2x0 C. to reduce a change per unit time in the thickness of nitride layers formed in the surfaces of the rings for thereby making it easier to control the thickness of the formed nitride layers.
However, when the molten salt containing 31-35% of CNOxe2x88x92 and 1-2% of CNxe2x88x92 is heated to 520-530xc2x0 C. to carry out the salt-bath nitriding process, a carbonate is separated out, and the composition of the salt bath changes easily. With the carbonate being separated out, the salt bath tank becomes shallow, making it impossible to fully dip the rings in the salt bath, and the temperature distribution in the salt bath becomes irregular, making uneven the thickness of the nitride layers and the aged hardness. The service life of the salt bath tank is shortened because the separated carbonate stores heat.
Another problem is that when the salt-bath nitride process is performed while the carbonate is being separated out, a white compound is separated out on the rings dipped in the salt bath.
It is therefore an object of the present invention to provide a method of manufacturing a laminated ring by easily producing rings having nitride layers of appropriate thickness.
To achieve the above object, there is provided a method of manufacturing a laminated ring for use as a belt in transmitting power in a continuously variable transmission, comprising the steps of welding opposite ends of a sheet of maraging steel to produce a plurality of rings, rolling the rings to a predetermined length, nitriding the rings in a salt-bath nitriding process by dipping the rings in a molten salt containing 38-46% of CNOxe2x88x92 and 1-2% of CNxe2x88x92 and heated to a temperature in the range from 480 to 530xc2x0 C., for a period of time ranging from 10 to 25 minutes, and stacking the rings into a laminated ring.
The molten salt can be prepared by melting, with heat, a molten salt composition which contains 3-4 of potassium cyanide (KCN) or sodium cyanide (NaCN), and 89-91% of potassium cyanate (KCNO) or sodium cyanate (NaCNO), and the remainder of potassium carbonate (K2CO3) or sodium carbonate (Na2CO3).
With the molten salt composition containing 38-46% of CNOxe2x88x92 and 1-2% of CNxe2x88x92, even when the temperature of the molten salt is in the range from 480 to 530xc2x0 C. which is lower than the conventional range from 570 to 580xc2x0 C., no carbonate is separated out. Therefore, the nitriding process on the surface of the ring can easily be controlled to produce a nitride layer of appropriate thickness and prevent a white compound from being separated out on the surface of the ring. The molten salt composition thus selected is effective to improve a carbo-nitriding action to produce a ring having excellent surface hardness and fatigue-resistant strength.
If CNOxe2x88x92 were less than 38%, then a carbonate might be separated out in the temperature range from 480 to 530xc2x0 C. If CNOxe2x88x92 were more than 46%, then the surface roughness of the ring might be increased to reduce the thickness of the nitride layer formed in the surface of the ring.
In the method according to the present invention, CNxe2x88x92 serves as a buffer. If CNxe2x88x92 falls out of the above range, it is impossible to keep CNOxe2x88x92 in the appropriate range.
If the temperature of the salt bath were less than 480xc2x0 C., then a carbonate might be separated out. If the temperature of the salt bath exceeded 530xc2x0 C., then the thickness of the nitride layer formed in the surface of the ring would change greatly per unit time, making it difficult to obtain a nitride layer of appropriate thickness. The temperature of the salt bath should preferably be in the range from 520 to 530xc2x0 C. to reliably prevent a carbonate from being separated out.
If the ring were dipped in the salt bath for a period of time less than 10 minutes, no desired aged hardness would be achieved. If the ring were dipped in the salt bath for a period of time longer than 25 minutes, then the thickness of the nitride layer formed in the surface of the ring would be excessive.
The rings are nitrided according to the salt-bath nitriding process such that the nitride layer formed in the surface of each of the rings has a thickness ranging from 20 to 40% of the overall thickness of the ring.
Because the thickness of the nitride layer ranges from 20 to 40% of the overall thickness of the ring, the expansion of the ring due to the formation of the nitride layer in the surface of the ring and the shrinkage of the ring due to the internal aging thereof are balanced to minimize changes in the circumferential length of the ring. If the thickness of the nitride layer were smaller than 20 of the overall thickness of the ring, then the shrinkage of the ring due to the internal aging thereof would be increased, tending to reduce the circumferential length of the ring. If the thickness of the nitride layer were greater than 40 of the overall thickness of the ring, then the expansion of the ring due to the formation of the nitride layer in the surface of the ring would be increased, tending to increase the circumferential length of the ring.
Since the rings manufactured by the method according to the present invention have small circumferential lengths variations and excellent dimensional stability, the rings have a desired level of tensile strength and fatigue-resistant strength.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.